I’ve had this same issue on Gentoo and now on Alpine, both with plasma 6 (Wayland). Pipewire and plasma 6 seem to be working as intended other than that… Any help would be appreciated!
E: the issue is plasma 6 exclusive, I have hyprland installed along side it and screensharing works just fine there
The KDE team has already determined that this is not a bug and that both you and me must just be imagining it:
https://bugs.kde.org/show_bug.cgi?id=481058